Computer-implemented method for determining centration parameters

ABSTRACT

Centration parameters for fitting spectacle lenses to a predetermined spectacle frame and to the head of a subject are determined with a computer-implemented method. At least two calibrated images, recorded from different recording directions, of the head of the subject wearing the spectacle frame are provided, wherein geometric parameters are established by geometric position determination. The geometric parameters describe the position of the eyes and the geometry of the spectacle frame, and the centration parameters are calculated from the geometric parameters. Further, a three-dimensional model for the spectacle lenses, which are to be received in the spectacle frame, is fitted to the geometry of the geometric parameters that describe the geometry of the spectacle frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of international patent application PCT/EP2018/051841, filed Jan. 25, 2018, which claims priority to European patent application EP 17153559.4, filed on Jan. 27, 2017, both of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The disclosure relates to a computer-implemented method for determining centration parameters, in particular for fitting spectacle lenses to a given spectacle frame and to the head of the subject, and/or for centring spectacle lenses, wherein at least two images of the head of the subject wearing a spectacle frame, which are calibrated to one another and recorded from different recording directions, are provided.

BACKGROUND

Centration parameters are used to correctly arrange or centre spectacle lenses in a spectacle frame, such that the spectacle lenses are arranged at the correct position relative to the eyes of the spectacles-wearing person. Here, these are in part anatomical parameters of the relevant person such as the interpupillary distance, in part purely frame-specific parameters such as the frame disk width or the frame disc height and in part combinations of anatomical and frame-specific parameters, such as the vertex distance and the fitting point height. An overview of conventional centration parameters is provided in DIN EN ISO 13666, dated October 2013.

Uncalibrated images are recorded in known methods for determining centration parameters, with a measuring frame having to be fastened to the spectacles or the spectacle frame. This is cumbersome and irritating. Further, US 2003/0123026 A1 discloses an apparatus of the type set forth at the outset, which automatically calculates centration parameters from images. In the known method, the spectacles are tried on virtually. This assumes that the parameters describing the geometry of the spectacle frame are known in advance.

SUMMARY

It is an object of the disclosure to develop a computer-implemented method of the type set forth at the outset in such a way that it is easier to carry out.

According to the disclosure, this object is achieved by a computer-implemented method wherein a three-dimensional model for the spectacle lenses to be received in the spectacle frame is fitted to the geometric parameters describing the geometry of the spectacle frame. In particular, planes or linear combinations of surfaces of n-th order are fitted to the parameters describing the geometry of the spectacle frame as an approximation for the surfaces of the spectacle lenses to be received in the spectacle frame and centration parameters are calculated from the geometric parameters describing the position of the eyes from the geometric parameters describing the geometry of the spectacle frame and from the three-dimensional model.

The disclosure is based on the concept of providing a simplified model for the spectacle lenses to be received in the spectacle frame by virtue of parameters describing the geometry of the spectacle frame and, in particular, parameters describing nasal and/or temporal frame edges being ascertained and the model for the spectacle lenses being fitted to these parameters. The frame edges can be approximated by straight lines.

In the process, provision is made of calibrated images. The calibration thereof comprises the extrinsic properties of the cameras recording the images or the camera recording the images in succession, such as the relative alignment of their optical axes and the relative alignment with respect to one another in space, and their intrinsic properties, i.e., the properties of the cameras themselves, which define how a point in space, which is situated in the internal coordinate system of the respective camera, is mapped onto the coordinates of the pixels of the recorded image. A detailed description of the calibration of cameras is found in the textbook entitled “Multiple View Geometry in Computer Vision” by Richard Hartley and Andrew Zisserman, 2nd edition, Cambridge University Press 2004, and, in particular, on page 8 therein. Geometric position determination is used to ascertain not only geometric parameters describing the position of the eyes from the images but also geometric parameters describing the geometry of the spectacle frame. Moreover, a three-dimensional model for the spectacle lenses to be received in the spectacle frame is fitted to the geometric parameters describing the geometry of the spectacle frame. The geometric parameters describing the position of the eyes, the geometric parameters describing the geometry of the spectacle frame and the three-dimensional model are used, as alternatives to one another or in combination, for the purposes of calculating the centration parameters.

An exemplary embodiment of the method provides for planes or linear combinations of surfaces of n-th order, in particular of at least one plane and/or at least one paraboloid, to be fitted to the parameters describing the nasal and/or temporal frame edges as an approximation for the surfaces of the spectacle lenses to be received in the spectacle frame. This represents a simplification of the model, which is easier to calculate. Expediently, the parameters describing the nasal and temporal frame edges are ascertained by means of epipolar geometry and/or by means of triangulation. Here, it is typical for at least three calibrated images of the head, which are recorded at the same time from different recording directions, to be provided, with a frontal image imaging the head from the front and a lateral image in each case imaging the head from the left and from the right. The recording directions of the lateral images each expediently include an angle of at least 60 degrees, e.g., 90°±10°, with the recording direction of the frontal image. The frontal and lateral images need not overlap in the method according to the disclosure. Regions of the images possibly overlapping with one another are expediently not used for determining the centration parameters.

According to an exemplary embodiment, provision is made for the geometric parameters describing the position of the eyes to include the position of the corneal vertex in space.

According to an exemplary embodiment of the disclosure, the position of the corneal vertex in space, determined to a first approximation, is subjected to a correction calculation. The type of correction calculation then depends on the way in which the position of the corneal vertex in space is determined to the first approximation.

It is possible within the scope of a so-called pupil-based evaluation for the position of the corneal vertex in space to be determined to a first approximation as a point of intersection of a view ray, tangential to the cornea, from a lateral camera recording the lateral image with a view ray, directed onto the pupil, from a frontal camera recording the frontal image. If this is the case, the position of the corneal vertex is typically calculated according to the formula a=q+μ*v+μ*w by means of the correction calculation, where a denotes the position vector of the corneal vertex in space after carrying out the correction calculation, q denotes the position of the corneal vertex to a first approximation, μ denotes an empirical value for the distance between the pupil centre and the corneal vertex, v denotes a unit vector in the spatial direction from the pupil centre to the frontal camera, and w denotes a unit vector in the recording direction extending through the centre of a corneal sphere.

Alternatively, it is also possible to undertake a reflection-based evaluation by virtue of a flash being produced, typically by means of an LED, when recording the images, wherein the position of the corneal vertex in space is determined to a first approximation as the position of the reflection point of the flash on the cornea. Proceeding from this first approximation, the position of the corneal vertex in the horizontal direction proceeding from the reflection point can typically be calculated with the correction calculation by adding Δx=+/−r*sin(½*(arccos z/a+arctan x/(z−v))) to the x-coordinate when the flash is produced by a light source arranged centrally in front of the face of the subject, where r denotes an empirical value for the corneal radius, a denotes the distance between the optical centre of a frontal camera recording the frontal image and the reflection point, x and z denote the x- and z-coordinates of the reflection point in a coordinate system with the origin at the optical centre of the frontal camera, wherein the recording direction of the frontal camera corresponds to the z-direction and the x-direction is horizontally orthogonal to the z-direction and, when observed in the z-direction, points to the right, and v denotes the distance between the light source producing the flash and the optical centre of the frontal camera in the z-direction. In addition or as an alternative thereto, the position of the corneal vertex in the y-direction proceeding from the reflection point can typically be calculated by means of the correction calculation by adding Δy=+/−r*sin ½*(arctan l/(d−v)), where r denotes an empirical value for the corneal radius, d denotes the distance between the optical centre of the frontal camera and the reflection point, v denotes the distance between the light source producing the flash and the optical centre of the frontal camera in the z-direction and l denotes the distance between the light source and the optical centre of the frontal camera in the y-direction, wherein the y-direction is orthogonal to the x-direction and to the z-direction and points upward in space.

Here, the plus sign in the x-direction should be applied when the corneal vertex of the left eye from the view of the subject is detected; the minus sign should be applied to the right eye from the view of the subject. In the y-direction, the plus sign should be applied when the light source emitting the flash is positioned at a lower height than the frontal camera; the minus sign should be applied when it is assembled at a greater height.

Typically, the pupil or the reflection point is detected by means of feature extraction and/or feature matching (feature comparison) and/or by means of machine learning by comparison with a multiplicity of data known in advance. This method step may be preceded by a face detection and/or a detection of facial features such as the eyes as a pre-processing step, in which there is a detection in respect of which image data belong to the face of the subject such that only these data are included in the detection.

In order to be able to undertake the determination of the corneal vertex in space for both eyes, it is common if provision is made of at least one calibrated third image that is recorded from a lateral position in respect of the head at the same time as the first image and the second image.

Typically, the computer-implemented method according to the disclosure is carried out using an apparatus for calculating the position of the corneal vertex as described in detail below.

The centration parameters, determined according to the disclosure, may typically be used for centring a spectacle lens in a spectacle frame and/or for grinding a spectacle lens into a spectacle frame. Here, in one method step, the at least one spectacle lens is centred in the spectacle frame using the centration parameters determined or the at least one spectacle lens is ground on the basis of the determined centration parameters for an arrangement in the spectacle frame. In this way, it is possible to produce spectacle lenses and spectacles.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described with reference to the drawings wherein:

FIG. 1A shows an apparatus for determining centration parameters in a perspective view;

FIG. 1B shows an apparatus for determining centration parameters in a view from the front;

FIG. 2 shows an illustration of the correction calculation in the case of a pupil-based determination of the position of the corneal vertex;

FIG. 3A shows an illustration of the correction calculation in the case of a reflection-based determination of the position of the corneal vertex relating to a correction in the x-direction;

FIG. 3B shows an illustration of the correction calculation in the case of a reflection-based determination of the position of the corneal vertex relating to a correction in the y-direction;

FIG. 4A shows a view from the front of a head wearing spectacles with approximated frame edges projected thereon;

FIG. 4B shows a view from the side of a head wearing spectacles with approximated frame edges projected thereon; and

FIG. 5 shows a schematic illustration of approximately determined lens planes.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The apparatus 10 illustrated in the drawing serves to determine centration parameters for fitting spectacles. It has a column 12 which, in a height-adjustable manner, carries a camera carrier 14 which, in turn, carries a number of cameras 16 a, 16 b. In a plan view, the camera carrier 14 is bent into an approximately circular shape and it extends between two free ends 18 which are arranged at a distance from one another. An inner face 20 of the camera carrier 14 encloses an interior 22, in which the head of a subject is situated when images are recorded by the cameras 16 a, 16 b, to the front, i.e., towards the column 12, and to the sides. In a direction extending between the free ends 18, the inner face 20 is bent in a concave manner and it has, for example, the form of a portion of a lateral face of a cylinder, wherein a cylinder may have a circular or oval base. A lifting device, not illustrated in any more detail, is arranged in the column 12 in order to be able to position the camera carrier 14 at the correct height in relation to the head of the subject, it being possible to move the camera carrier 14 up-and-down in a motor-driven manner by way of the lifting device.

All cameras 16 a, 16 b are arranged in a camera arrangement 26 that extends between the free ends 18. In the present exemplary embodiment, the camera arrangement 26 is embodied as a camera row 26, the cameras 16 a, 16 b of which are all situated at the same height, with their optical axis being directed into the interior 22. In the present exemplary embodiment, the camera row 26 comprises a frontal camera 16 a arranged in the centre of the camera carrier 14, the optical axis of the frontal camera being directed frontally onto the face of the subject, and eight lateral cameras 16 b that are arranged symmetrically in pairs in respect of a perpendicular plane of symmetry extending through the optical axis of the frontal camera 16 a, four of the lateral cameras being directed onto the face of the subject from the left and four being directed onto the face of the subject from the right in each case. Moreover, the cameras 16 a, 16 b are calibrated in such a way that they are able to record calibrated images of the subject at the same time. The calibration comprises the extrinsic properties, such as the relative alignment of the optical axes and the relative arrangement with respect to one another in space, and their intrinsic properties, i.e., the properties of the cameras themselves, which define how a point in space, which is situated in the internal coordinate system of the respective camera, is mapped onto the coordinates of the pixels of the recorded image.

The camera carrier 14 only encloses the interior 22 to the front, towards the column 12, and to the sides, i.e., to the left and right of the head of the subject. Toward the top, the bottom and toward a rear side 30, the interior is open, wherein the free ends 18 have a distance from one another of at least 25 cm such that the subject can comfortably approach from the rear side. The distance is 70 to 80 cm in the shown exemplary embodiment.

In order to illuminate the interior 22, provision is made of an illumination device having an upper light strip 32 extending above the camera row 26 and a lower light strip 34 extending below the camera row 26, the light strips having a multiplicity of LEDs as lighting means in each case. The upper light strip 32 and the lower light strip 34 each extend, continuously or with interruptions, over a length which is at least as long at the length of the length of the camera row 26 as measured in the circumferential direction between the free ends 18. This corresponds to a circumferential angle of at least 160 degrees. Near the free ends 18, the upper light strip 32 and the lower light strip 34 are connected to one another, in each case by a further light strip 36 that extends in the vertical direction. Consequently, the camera row 26 is framed in the entirety thereof by at least one row of LEDs. The apparatus 10 moreover has an open-loop or closed-loop control device, not illustrated in any more detail in the drawing, by means of which the light intensity emitted by the LEDs can be controlled or regulated depending on the light intensity detected by the cameras 16 a, 16 b. Here, the LEDs of the light strips 32, 34, 36 are combined into sectors, the emitted light intensities of which can be controlled or regulated separately from one another. Moreover, provision is made for the light intensities emitted by the individual LEDs also to be able to be controlled or regulated separately from one another with the open-loop or closed-loop control device.

In order to be able to correctly position the subject in the interior 22, the two lateral cameras 16 b closest to the frontal camera 16 a are configured to measure the distance of the head of the subject from the centre 38 of the camera carrier 14. The subject is shown whether or not they are standing correctly by means of a display unit, which is not displayed in any more detail. The display unit has a plurality of differently coloured light sources arranged in a row. The central light source lights up green when the subject stands correctly. Proceeding from the central light source, there is, in this sequence, a green, an orange and a red light source in each direction, the light sources indicating, in accordance with their colour, if the subject is slightly, clearly or much too far away from the centre 38 of the camera carrier 14 or if the subject stands slightly, clearly or much too close to the centre 38. To ensure that the recording direction of the subject is directed to infinity when determining the centration parameters, a fixation device 42 arranged at the camera carrier 14 is provided, the fixation device producing a fixation pattern in the form of a speckle pattern for the subject. The fixation pattern is arranged slightly higher than the frontal camera 16 a so that the subject peers over the latter. In this way, their face can be recorded to the greatest possible extent.

The apparatus 10 is also suited, in particular, to produce an avatar of the head of the subject, which may be used for determining the centration parameters. To this end, calibrated images of the head of the subject without spectacles or spectacle frame are recorded by the cameras 16 a, 16 b. A depth profile of the head, which images the latter very well as an approximation, is created from the images by means of a suitable process for geometric position determination, such as triangulation. The head is imaged by a multiplicity of points which can be connected to one another with a mesh pattern or else be stored as a point cloud. In the subsequent determination of the centration parameters, the avatar thus ascertained may be used to determine centration parameters which cannot be determined, or can only be determined approximately, on account of the geometric properties of the spectacles or the spectacle frame worn by the subject. By way of example, a wide frame side may cover the eye in a lateral recording to such an extent that the vertex distance cannot be determined or can only be determined very inaccurately. Moreover, tinted or strongly reflecting spectacles may not allow the eyes to be identified, or only be identified very poorly. To counteract this, the depth profile of the avatar is projected onto the images, recorded by the cameras 16 a, 16 b, of the subject wearing the spectacles or spectacle frame and the centration parameters, which can only be determined very unsatisfactorily on account of the sight being restricted by the spectacles or spectacle frame, are determined by means of the image data of the avatar. Here, for the purposes of minimizing deviations, the avatar may be fitted to the images of the subject wearing the spectacles or spectacle frame.

The above-described apparatus 10 can be used as follows for both a pupil-based detection of a corneal vertex and for a reflection-based detection of a corneal vertex in both eyes of the subject.

In the pupil-based method according to FIG. 2, the position of the corneal vertex in space is initially determined to a first approximation as the point of intersection q of a first view ray 52, tangential to the cornea 50, from one of the lateral cameras 16 b recording a lateral image of the subject with a second view ray 56, directed onto the pupil 54, from a frontal camera 16 a recording a frontal image of the subject. A corrected position of the corneal vertex in space is calculated with a correction calculation using the equation a=q+μ*v+μ*w. Here, μ is an empirical value for the distance between the pupil centre and the corneal vertex, which typically assumes values between 2.5 mm and 4 mm. v is a unit vector in the spatial direction from the pupil centre p to the frontal camera 16 a, the coordinates of which are specified using the variable c1, and it is calculated as v=(p−c1)/|p−c1|. w is a unit vector of the recording direction extending through the centre m of the corneal sphere, the recording direction being directed onto the fixation pattern of the fixation device 42 at the spatial point t, and it is calculated as w=(t−m)/|t−m1|. All values a, q, p, c1, t, and m are three-dimensional vectors.

Two correction calculations have to be undertaken in the reflection-based determination of the position of the corneal vertex according to FIGS. 3A and 3B, wherein the first correction calculation (FIG. 3A) relates to a correction in the x-direction and the second correction (FIG. 3B) relates to a correction in the y-direction. These spatial directions are set by an internal coordinate system of the frontal camera 16 a, which has its origin in the optical centre of the front camera 16 a. Here, the z-direction is set by the recording direction of the frontal camera 16 a, the x-direction is a direction which extends horizontally and orthogonally with respect to the z-direction and which points to the right when observed in the direction of the latter, and the y-direction extends orthogonally to the x-direction and to the z-direction and points upward in space. In the case of the reflection-based measurement, a flash is emitted by means of a light source, an LED 58 in the present case, the reflection of the flash on the cornea being detected by the frontal camera 16 a and at least one of the lateral cameras 16 b and forming the first approximation for the position of the corneal vertex in space. In FIGS. 3A, 3B, the reflection point is denoted “approx”. In the x-direction, a correction is undertaken by adding Δx=+/−r*sin(½*(arccos z/a+arctan x/(z−v))) to the x-coordinate of the reflection point approx, wherein the plus sign should be used in an application to the left eye and the minus sign should be used in an application to the right eye (cf. FIG. 3a ). Here, r is an empirical value for the corneal radius, which is typically approximately 8 mm. a is the distance of the optical centre of the frontal camera 16 a from the reflection point approx and v is the distance of the LED 58 from the optical centre of the frontal camera 16 a in the z-direction. Once again, x and z are the coordinates in the x- and z-directions.

In the y-direction, a correction is undertaken by adding Δy=+/−r*sin ½*(arctan l/(d−v)), proceeding from the reflection point approx. Once again, r is the empirical value for the corneal radius, d is the distance of the optical centre of the frontal camera 16 a from the reflection point approx in the z-direction, v is the distance of the LED 58 from the optical centre of the frontal camera 16 a in the z-direction and l is the distance of the LED 58 from the optical centre of the frontal camera 16 a in the y-direction. The plus sign is used when the LED 58 is arranged below the frontal camera 16 a, i.e., if the y-coordinate of the LED 58 is smaller than the y-coordinate of the frontal camera 16 a or of the optical centre of the latter. The minus sign is used if the LED is arranged above the frontal camera 16 a.

In the above-described method, the pupil or the reflection point approx can be detected, for example, by means of feature extraction and/or feature matching and/or by means of machine learning by comparison with a multiplicity of data known in advance. This detection step can be preceded by a step in which a face detector identifies which pixels belong to the face of the subject or to their eye area such that there can already be a restricted search for the pupil or the reflection point approx.

The position of the corneal vertex in space is used for determining the centration parameters when fitting the spectacles. By means of the calibrated images recorded by the cameras 16 a, 16 b, geometric parameters describing the geometry of the spectacle frame are ascertained by geometric position determination, in particular by triangulation or epipolar geometry. The parameters comprise the nasal and temporal frame edges 60, 62, as indicated in FIGS. 4A and 4B in exemplary fashion. Planes 64 are fitted to the parameters describing the nasal and temporal frame edges 60, 62, as an approximation for the surfaces of the spectacle lenses to be received in the spectacle frame. FIG. 5 schematically shows the planes 64 in front of the respective, approximately represented cornea 66 with schematically illustrated visual beams 68 of the cameras 16 a, 16 b. Finally, the centration parameters are calculated from the data obtained.

The foregoing description of the exemplary embodiments of the disclosure illustrates and describes the present invention. Additionally, the disclosure shows and describes only the exemplary embodiments but, as mentioned above, it is to be understood that the disclosure is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art.

The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “having” or “including” and not in the exclusive sense of “consisting only of.” The terms “a” and “the” as used herein are understood to encompass the plural as well as the singular.

All publications, patents and patent applications cited in this specification are herein incorporated by reference, and for any and all purposes, as if each individual publication, patent or patent application were specifically and individually indicated to be incorporated by reference. In the case of inconsistencies, the present disclosure will prevail. 

1. A computer-implemented method for determining centration parameters, the method comprising: providing at least two images of a head of a subject wearing a spectacle frame, the at least two images being calibrated to one another and recorded from different recording directions; ascertaining geometric parameters describing a position of eyes of the subject from the at least two images by geometric position determination; fitting a three-dimensional model for spectacle lenses to be received in a spectacle frame to geometric parameters describing a geometry of the spectacle frame, wherein at least one of planes or linear combinations of surfaces of n-th order are fitted to the geometric parameters describing the geometry of the spectacle frame as an approximation for the surfaces of the spectacle lenses to be received in the spectacle frame; and calculating centration parameters from the geometric parameters describing the position of the eyes, the geometric parameters describing the geometry of the spectacle frame, and the three-dimensional model.
 2. The computer-implemented method according to claim 1, wherein the surfaces of n-th order comprise a plane, a paraboloid, or the plane and the paraboloid.
 3. The computer-implemented method according to claim 1, wherein a plane is respectively fitted to the parameters describing the geometry of the spectacle frame as an approximation for the surface of each of the spectacle lenses to be received in the spectacle frame or each spectacle frame edge receiving the spectacle lenses.
 4. The computer-implemented method according to claim 1, wherein the at least two images are recorded at a same time.
 5. The computer-implemented method according to claim 1, wherein the geometric parameters describing the geometry of the spectacle frame include parameters describing nasal and/or temporal frame edges approximated by straight lines, and wherein the three-dimensional model is fitted to the parameters describing the nasal and/or temporal frame edges.
 6. A computer-implemented method for determining centration parameters, the method comprising: providing at least two images of a head of a subject wearing a spectacle frame, the at least two images being calibrated to one another and recorded from different recording directions; ascertaining geometric parameters describing a position of eyes of the subject from the at least two images by geometric position determination; fitting a three-dimensional model for spectacle lenses to be received in the spectacle frame to geometric parameters describing a geometry of the spectacle frame, wherein the geometric parameters describing the geometry of the spectacle frame include parameters describing a nasal frame edge, a temporal frame edge, or the nasal frame edge and the temporal frame edge; and calculating centration parameters from the geometric parameters describing the position of the eyes, the geometric parameters describing the geometry of the spectacle frame, and the three-dimensional model.
 7. The computer-implemented method according to claim 6, wherein at least one of planes or linear combinations of surfaces of n-th order are fitted to the parameters describing the nasal frame edge, the temporal frame edge, or the nasal frame edge and the temporal frame edge as an approximation for surfaces of the spectacle lenses to be received in the spectacle frame.
 8. The computer-implemented method according to claim 1, further comprising: providing at least three images of the head of the subject wearing the spectacle frame, the at least three images being calibrated to one another and recorded at a same time from different recording directions, wherein a frontal image images the head from a front side, a left lateral image images the head from a left side, and a right lateral image images the head from a right side.
 9. The computer-implemented method according to claim 8, wherein the recording directions of the left and right lateral images each include an angle of more than 60 degrees and no more than 120 degrees with the recording direction of the frontal image.
 10. The computer-implemented method according to claim 1, wherein the at least two images do not overlap or wherein the centration parameters are determined only from non-overlapping regions of the at least two images.
 11. The computer-implemented method according to claim 1, wherein the geometric parameters describing the position of the eyes include a position of a corneal vertex in space.
 12. The computer-implemented method according to claim 11, further comprising: performing a correction calculation of the position of the corneal vertex.
 13. The computer-implemented method according to claim 11, further comprising: determining the position of the corneal vertex in space to a first approximation as a point of intersection q of a first view ray, tangential to the cornea, from a lateral camera recording the lateral image with a second view ray, directed onto the pupil, from a frontal camera recording the frontal image.
 14. The computer-implemented method according to claim 11, wherein the correction calculation comprises calculating the corneal vertex according to a=q+μ*v+μ*w, where a denotes the position vector of the corneal vertex in space after carrying out the correction calculation, q denotes the position of the corneal vertex to a first approximation, μ denotes an empirical value or an actual value for the distance between the pupil centre and the corneal vertex, v denotes a unit vector in the spatial direction from the pupil centre to the frontal camera, and w denotes a unit vector in the recording direction extending through the centre of a corneal sphere.
 15. The computer-implemented method according to claim 1, further comprising: producing a flash when recording the images, and determining the position of the corneal vertex in space to a first approximation as the position of the reflection point of the flash on the cornea.
 16. The computer-implemented method according to claim 11, wherein the position of the corneal vertex in a horizontal direction proceeding from the reflection point is calculated with the correction calculation by adding Δx=+/−r*sin(½*(arccos z/a+arctan x/(z−v))) to the x-coordinate, where r denotes an empirical value or an actual value for the corneal radius, a denotes the distance between the optical centre of a frontal camera recording the frontal image and the reflection point, x and z denote the x- and z-coordinates of the reflection point in a coordinate system with the origin at the optical centre of the frontal camera, wherein the recording direction of the frontal camera corresponds to the z-direction and the x-direction is horizontally orthogonal to the z-direction and, when observed in the z-direction, points to the right, and v denotes the distance between the light source producing the flash and the optical centre of the frontal camera in the z-direction.
 17. The computer-implemented method according to claim 11, wherein the position of the corneal vertex in the y-direction proceeding from the reflection point is calculated with the correction calculation by adding Δy=+/−r*sin ½*(arctan l/(d−v)), where r denotes an empirical value or an actual value for the corneal radius, d denotes the distance between the optical centre of the frontal camera and the reflection point in the z-direction, v denotes the distance between the light source producing the flash and the optical centre of the frontal camera in the z-direction and l denotes the distance between the light source and the optical centre of the frontal camera in the y-direction, wherein the y-direction is orthogonal to the x-direction and to the z-direction and points upward in space.
 18. A computer program stored on a non-transitory storage medium with program code for carrying out the method according to claim 1 when the computer program is loaded onto a computer and/or executed on the computer.
 19. A method of operating an apparatus for carrying out the computer-implemented method according to claim 1, wherein the apparatus has a camera carrier configured to partly enclose an interior that is open to the top, to the bottom, and to a rear side, wherein the camera carrier carries at least three cameras that are arranged between two free ends of the camera carrier and directed toward the interior, and wherein the camera carrier has an illumination device configured to illuminate the interior.
 20. A method for centring at least one spectacle lens in a spectacle frame, the method comprising: determining centration parameters of the at least one spectacle lens with the method according to claim 1; and centring the at least one spectacle lens in the spectacle frame with the determined centration parameters.
 21. A method for grinding at least one spectacle lens into a spectacle frame, wherein centration parameters are determined with the method according to claim 1, and wherein the at least one spectacle lens is ground on the basis of the determined centration parameters for an arrangement in the spectacle frame.
 22. A method for producing a spectacle lens comprising: grinding the spectacle lens into a spectacle frame according to the method of claim
 21. 23. A method for producing spectacles, the method comprising: centring at least one spectacle lens in a spectacle frame according to the method of claim
 20. 24. An apparatus for determining centration parameters for fitting spectacle lenses to a given spectacle frame and to a head of a subject, the apparatus comprising: a first camera configured to record a first image from a first viewing direction; at least one second camera, calibrated to the first camera, configured to record a second image from a second viewing direction that differs from the first viewing direction; and a computer having a processor configured to execute a computer program and a non-transitory memory in which the computer program is stored, the computer program having program code that, when executed by the processor, causes the apparatus to: provide at least two images of the head of a subject wearing a spectacle frame, which are calibrated to one another and recorded from different viewing directions, ascertain geometric parameters describing the position of the eyes and geometric parameters describing the geometry of the spectacle frame, from the images by geometric position determination, calculate centration parameters from the geometric parameters, on the basis of the centration parameters, and fit a three-dimensional model for the spectacle lenses to be received in the spectacle frame to the geometric parameters describing the geometry of the spectacle frame. 